1. Field of the Invention
This invention relates to the field of insulated coated heaters for indirectly heated cathodes of electron discharge tubes and especially to thermally darkened coated heaters.
2. Description of the Prior Art
Examples of dark coated insulated heaters are shown in U.S. Pat. Nos. 3,195,004, 3,246,197, 3,328,201 and 3,450,565. As disclosed in said patents, a dark coating is obtained by mixing particulate alumina (aluminum oxide) with tungsten, the tungsten being either in particulate elemental form or in solution. When the tungsten is used in solution form, it is reduced to the dark elemental state, either after it has been deposited on the aluminum oxide particles or after it has been deposited on a coated heater.
As pointed out in said patents, the purpose of the dark coating is to electrically insulate the heater from the cathode into which it is inserted and to improve the efficiency of heat transfer from the heater to the cathode, a dark coating being more efficient in heat transfer than the usual white alumina coating.
In the heater coatings of the prior art, it is difficult to obtain uniformity of dark color from batch to batch of dark coating.